fix: correct character limit calculation in changelog truncation

Address Copilot review feedback to properly account for the extra
message length when truncating long changelogs.

Changes:
- Extract extra_message into a variable for accurate length calculation
- Calculate max_length by subtracting both feedback_link and extra_message lengths from 4000 limit
- Ensures total output never exceeds TestFlight's 4000 character limit

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: graycreate

fastlane/changelog_helper.rb

@@ -94,8 +94,9 @@ def self.format_for_testflight(content)
 
     # Ensure we don't exceed TestFlight's changelog length limit (4000 chars)
     if (formatted + feedback_link).length > 4000
-      max_length = 3900 - feedback_link.length
-      formatted = formatted[0...max_length] + "\n\n(See full changelog at github.com/v2er-app/iOS)"
+      extra_message = "\n\n(See full changelog at github.com/v2er-app/iOS)"
+      max_length = 4000 - feedback_link.length - extra_message.length
+      formatted = formatted[0...max_length] + extra_message
     end
 
     formatted + feedback_link